The present invention relates to internal structures of rotary kilns, and more particularly to trefoil structures in rotary kilns, and even more particularly to preformed, modular trefoils and installation methods for the same.
A rotary kiln is a long refractory-lined cylinder that thermally treats material as its flows from its upper, feed end to its lower, outlet end. The kiln is slightly inclined and rotates about its longitudinal axis to promote material flow. Most kiln processes are counter-current such that the hot gas flows from the material outlet end to the material inlet end. The kiln includes a steel shell having a refractory lining on its inside surface. For larger kilns, the refractory lining typically includes a refractory brick lining. Rotary kilns generally operate on a twenty four hour basis for several months between scheduled down periods.
Rotary kilns are employed for calcining limestone, calcining and sintering dolomite and magnesite, lime re-burning in paper plants, processing cement, calcining petroleum coke, various incineration processes, and similar thermal processes. In a lime manufacturing process, coarse limestone is fed into the feed end of the kiln. As the limestone feed tumbles down the kiln, it is dried and then calcined into lime by the hot gases.
Rotary kilns may employ internal heat exchanger structures, such as refractory trefoils or metallic heat exchanges that divide the cross section of the kiln into three or more segments to enhance the heat transfer from the gas to the material, improve mixing of the material, and provide similar benefits. Although trefoils enhance heat transfer from the gas to the material, conventional trefoils constrict the overall area through which the counter-current air stream may flow. Such a constriction is an undesirable design limitation of the trefoil because the constriction increases the pressure in the burning zone and the air velocity in the trefoil area, therefore affecting the flame burning characteristics and heat transfer, and may also increase the dust load carried by the air stream. The weight of current refractory trefoil designs is considerably more per foot if rotary kiln than a single layer brick lining, and thus exerts additional mechanical stress on the kiln shell.
Trefoils within a rotary kiln encounter harsh operating conditions. For example, internal gas temperatures may typically be 1000 to 3000 degrees F. in a highly basic atmosphere in a rotary lime kiln, although temperatures outside of this range are possible depending on the particular application. The trefoil must take the structural loading and erosion from several hundred tons per day of partially calcined rock that slides across or falls against the surfaces of the trefoil. The trefoil is continuously rotated with the kiln, which subjects the trefoil components to varying compressive and tension force. Further, the trefoil must withstand the kiln shell deflection upon revolution over its roller supports. The trefoil is critical to the operation of the manufacturing facility--often failure of a trefoil during operation requires the entire manufacturing facility to be shut down for repairs. Without the trefoil's improved heat exchange, product sintering may be inadequate. Many kilns also employ expensive metallic heat exchangers, which require refractory trefoil heat exchangers "down kiln" of them to avoid damage from high gas temperatures. Trefoils generally reduce fuel consumption and also government-regulated stack emissions. Failure of a trefoil may therefore cause a rotary kiln plant to become "non-compliant", leading to a shut-down or significant monetary penalties.
Conventional trefoils typically are from 9-15 feet long along the longitudinal kiln axis, depending on the kiln diameter and other parameters, and having "spokes" or legs typically from, 9-12" thick. A refractory trefoil often obtains the vast majority of its heat exchange benefits in about the first 3 inches of material thickness beneath the surfaces exposed to the heat. A trefoil "leg" is exposed to hot gasses and material on two faces during each revolution; thus trefoil thicknesses over about 6 inches are unnecessary for the heat exchange function. Conventional trefoils employ leg thicknesses from about 9-12 inches primarily to provide mechanical stability within the severe rotary kiln environment. These thicknesses have been found to be needed because of tendency of conventional bricks to shift from proper alignment and thus fail prematurely and from the inability to obtain satisfactory strength from "in-situ" cast and cured monolithic trefoils.
Conventional trefoils typically are formed from individual (usually interlocking) refractory bricks, although some were formed from "in-situ" cast and cured monolithics. The manufacturing process for producing bricks includes high pressure pressing, often at 15,000 to 20,000 pounds per square inch (PSI), and firing, often up to approximately 2,400 degrees F. (or higher). Bricks produced by pressing and firing typically have high density, low porosity, good volume stability upon heating, and high mechanical strength at standard and elevated temperatures. However, brick size and complexity of shape axe limited by the mechanical limitations of pressing and handling equipment.
Brick trefoils, therefore, generally employ small standard, interlocking shapes that require specially engineered and formed shapes to form contours at the shell and near the hub. The limitations of brick technology generally require leg thicknesses greater than about the 6 inches optimum for heat transfer. Installation is labor-intensivle and requires specially skilled artisans to form the trefoil. They also require complicated forms (specific to a single rotary kiln size) to support them during construction. Thus, brick trefoils are slow to install and are expensive.
Further, technical considerations of trefoil design include the kiln diameter, kiln ovality, expected kiln deflection, expansion or contraction characteristics of the brick upon heating, kiln internal temperature range, and type of product.
For example, a particular design concern is the choice of the number of joints that form the trefoil leg. The joints enable a small amount of flexing, for example upon kiln shell deflection during rotation, which increases the elasticity and diminishes excessive mechanical stress of the brick trefoil leg. However, the working of adjacent bricks, which may cause wear and failure, counter-balances the benefit of increased elasticity. Thus, an appropriate number and design of brick trefoil joints, which is mostly based on empirical knowledge, balances these factors
U.S. Pat. No. 5,330,351, entitled "Trefoil Construction For Rotary Kilns" ("Ransom") discloses a trefoil which has legs that are each formed from four basic, precast shapes assembled in the kiln. Several blocks of some of the types of shapes are employed to form the trefoil. Conventional brick trefoils generally include shapes that interlock, including, for example, tongue-and-groove type interlocking pieces, as disclosed for example in the '351 patent (Ransom). The interlocking shapes prevent or limit relative movement of the bricks, which may subject the interlocking parts to shear forces. Because of the high strength required of the protruding portions, among other factors, the interlocking bricks or shapes employed in rotary kiln trefoils generally must have a high hot modulus of rupture (HMOR). For example, the '351 patent (Ransom) discloses ultra-high strength castable having a HMOR of 3000 PSI at 2500 degrees F.
Other examples of conventional trefoils include U.S. Pat. No. 3,030,091, entitled, "Rotary Kiln with Heat Exchanger" ("Witkin") which discloses a rotary kiln having a trefoil heat exchanger with each section having a dam at the downstream end. Further, U.S. Pat. No. 3,036,822, Entitled,"Rotary Kiln with Built-in Heat Exchanger" ("Anderson") discloses a rotary kiln with partitions dividing the material stream into six segments. U.S. Pat. Nos. 3,169,016 and 3,175,815, entitled "Kiln" ("Witken") disclose a trefoil having apertures that enables material to drop into an adjacent chamber to enhance heat transfer. U.S. Pat. No. 4,846,677, entitled, "Castable Buttress for Rotary Kiln Heat Exchanger and Method of Fabricating" ("Crivelli") discloses a trefoil rotary kiln with buttressed end portions of poured-in-place cast refractory to prevent the trefoil from sliding downhill during kiln rotation.
Within the past 30 years, in-situ cast monolithic refractory trefoils have been installed in commercial rotary kilns. However; because of premature wear, complicated forms, and slower installation than brick, "in-situ" casting quickly became typically commercially untenable. In-situ casting includes building forms within the kiln that are attached to the kiln shell. A first form having a height less than the kiln radius is erected at the bottom dead center of the kiln. After castable refractory is mixed with water and poured into the first form, and after a waiting period of from 18 to 36 hours is allowed for setting, the kiln is rotated by 120 degrees (for a three-leg trefoil) and the first form is supported by temporary bracing. Castable refractory is poured into a second form erected and braced like the first, and the kiln is rotated another 120 degrees for pouring castable refractory in a third form. A hub form is erected to join the innermost ends of the castable members, and castable refractory is poured within the hub form. Often after a day of air-drying, the forms are removed and the kiln is heated slowly according to a drying and curing schedule of the castable refractory.
FIG. 6 (Prior Art) shows a cross sectional view of a portion of a castable trefoil 110 during forming. Partially formed trefoil 110 has three forms 108A, 108B, and 108 C filled with castable refractory 112A, 112B, and 112C, respectively, with the hub form 109 ready to receive castable refractory. The cast structure is secured to the kiln shell 106 by v-shaped anchors, which are not shown. The rotary kiln brick 107 is shown schematically, and the brick 107 will abut the refractory 112A, 112b, and 112C to cover the interior surface of the kiln shell 106 after the forms 108A, 108B, and 108C are removed.
Although less expensive than brick trefoils, "in-situ" cast trefoils tend to have a shorter life than brick trefoils for three main reasons. First, the lack of joints create excessive mechanical stress from the rotation and deflection of the kiln shell, and from thermal factors. Second, castable refractory products generally do not match brick products in strength or thermal properties unless cast/cured under tightly controlled conditions. Third, because a rotary kiln can not be rotated at full speed in a cold state because of the risk of the brick lining being dislodged from the shell but must be rotated when hot (to prevent sagging of the steel shell); a very rapid heat-up schedule is typically used, which forces a castable trefoil to undergo a much shorter than optimum curing period.
Additional disadvantages of the cast in-situ method include: the need to handle, assemble, and disassemble bulky molds inside the rotary kiln; difficult curing of the refractory monolith during the burn-in of the rotary kiln; and difficulties working with wet materials in sub-freezing temperatures.
Regardless of how the trefoil is formed, trefoil installation and maintenance generally require the kiln, and thus the entire manufacturing facility, to be shut down for several days. For example, an operational rotary lime calcining kiln may require one or two days to cool the system from its operating temperature just to enable personnel access. The extensive time required for installing a brick trefoil or a forming a cast trefoil adds downtime and cost.
It is a goal of the present invention to provide a trefoil that is easy or cost effective to produce and install and that has good mechanical and structural properties, and to provide method of installing the trefoil.